Everyday, the light-sensitive cells in our retinas, our photoreceptors, shed and renew a portion of their light-sensitive membrane. Why this process occurs is not completely understood, however; it is essential for healthy vision in humans and animals. In fact, retinal degeneration occurs in mutant rats unable to shed their photosensitive membrane. In most animals, shedding of the photosensitive membrane is regulated by a biological clock (circadian rhythm) and triggered by that clock and/or by light. Relatively little is known, however, about how the biological clock and light interact to trigger and regulate daily shedding and renewal of photosensitive membrane. The lateral eye of the horseshoe crab (Limulus) has long served as an important system for the study of photoreceptor membrane shedding and renewal. Its photoreceptors are similar in many important ways to human rod photoreceptors. In the currently funded project (R15 EY13196), we developed and tested a model for the interaction of light and the biological clock in the regulation of daily transient photosensitive membrane shedding. To date, we have learned that the clock primes the retina for transient shedding and renewal through cAMP-dependent protein kinase (PKA) stimulated downregulation of photoreceptor protein synthesis. This competing continuation application proposes to continue to evaluate our model through the following two specific aims: (1). Investigate whether downregulation of visual arrestin expression is required to prime the lateral eye for transient rhabdom shedding (TRS), and whether the actin-binding protein myosin III is required to stabilize the rhabdom against TRS during post-dawn daylight. (2). Determine whether PKA acts through, or in competition with, a MAP kinase pathway to decrease photoreceptor protein synthesis during octopaminergic priming of the lateral eye for TRS. The P.I. and his undergraduate student collaborators will use gene silencing through the RNA interference pathway to knockdown the expression of proteins (arrestin and myosin III). Our model hypothesizes that the clock regulates the expression of these proteins, and possibly others, to maintain healthy vision through proper regulation of daily turnover of the photosensitive membrane. Public Health Significance - The use of a simple invertebrate retina with many similarities to the human retina is critical to the development of translational models that can be used to drive research to prevent retinal degeneration and blindness caused by failures in daily photoreceptor outer segment turnover. Involving undergraduate students in biomedical research is critical to this Nation's ability advance our public health through recruitment of our brightest young citizens into medicine and biomedical research, and to produce new generations of well-informed science teachers and College professors to inform the Citizenry. [unreadable] [unreadable] [unreadable]